With iMessage, we’ve created a new messaging service for all iOS 5 users. You can send unlimited text messages via WiFi or 3G from your iPad, iPhone or iPod touch to anyone with one of those devices. iMessage is built into the Messages app so you can send text, photos, videos, locations and contacts. Leep everyone in the loop with group messaging. Track your messages with delivery receipts and optional read receipts, see when someone’s typing and enjoy secure encryption for text messages. Even start a conversation on one of your iOS devices and pick up where you left off on another.That pitch leaves a lot of questions unanswered. For example, do iMessages count against my text messaging plan? Where do I sing up for iMessage? What if the person on the other end cannot receive iMessages? What about sending iMessages to non-Apple devices? Here’s what we have found out so far…

For starters, iMessage is a proprietary protocol designed by Apple, meaning you cannot exchange iMessages with your BlackBerry- or Android-toting friends. True, Apple could talk to standards bodies and make iMessage an open platform. That said, however, we all remember too well the company promised to do the same with FaceTime and they still haven’t worked out deals with carriers to enable FaceTime video calling over 3G.

On the upside, iMessages is pretty seamless. It’s built right into the standard Messages app so users needn’t learn anything – they just send a message. When you first install iOS 5 and enable the iMessage service, your phone number and Apple ID are automatically registered with the service. This lets anyone ping you on iMessage via either your phone number (for iPhone users) that your contacts already have in their address book or your email address, which is what iPad and iPod touch users might prefer to use.

Put simply, iMessage lets iOS 5 users communicate between themselves for free, avoiding costly SMS messages. And because it’s built into iOS 5 and really simple to use, iMessage is bound to take off pretty darn quickly. As a reader noted in the comments, “many people (including elderly folks) will use iMessage without really knowing”. Come again? Well, even pinging someone you never contacted before will automatically convert a regular SMS message to an iMessage because iOS 5 can determine whether any two parties can exchange iMessages.

This actually happens the instant you type in the recipient’s phone number (or choose an address book contact to message), iOS 5 talks in the background to Apple’s servers in order to determine whether or not that person runs iOS 5 and has the iMessage service enabled in their Settings. If not, the message gets delivered as a regular SMS or MMS message, depending on its contents. This is denoted by the green Send button, in which case the message counts against your monthly SMS allowance and could possibly incur charges from your carrier. If the Send button turns blue and the gray “iMessage” label appears inside the text entry field, you are sending an iMessage instead of a regular text message. Also handy, when you create a new message and begin typing in the recipient’s name, the auto-complete list shows the blue chat bubble next to address book contacts that have iMessage enabled.

Sending an SMS message (left) and an iMessage (middle) – check out the difference in color coding of the recipient field and the Send button. The blue chat bubble (right) helps me figure out if a contact is iMessage-enabled.

Best of all, iOS 5 syncs your messages via the cloud across all your devices. I’ve tested this by chatting with a friend using my iPhone 4 and picking up the conversation later on my iPad – works as advertised. You can set certain aspects of the iMessage service in the Messages section of the iOS 5 Settings app. Here you can disable iMessages altogether and go with regular text messages instead. Other preference settings include read receipts, delivery reports, setting your caller ID (your phone number or an email address) and register one or more email addresses where people could send you iMessages. There are couple of unknowns as well. Considering iMessages are sent over WiFi or 3G, it’s unknown whether they count against your monthly 3G data allowance or if Apple has reached a deal with carriers to exclude iMessages from yuor data traffic. This would be especially critical for users abroad, to avoid exorbitant roaming data charges incurred by exchanging iMessages worldwide.

You can disable iMessages or SMS messages in Settings, register additional email addresses with iMessages beyond your Apple ID and even change your caller ID from your phone number to an email address.

We are also wondering if Apple will updated iChat in Lion with iMessage support. And while we’re at it, how about iChat for Windows to enable cross-platform communication between Macs, PCs and iOS devices? Last, but certainly not the least, reader Sagar hinted at a keynote slide that suggests emoticons support in iMessage, adding that the iOS 5 features list “also points they’re adding Emoji emoticons as an addon to iOS 5″, which is pretty neat.

Check out the emoticon in an iMessage on a slide behind Scott Forstall. iOS 5 is also said to provide optional Emoji emoticons.